Chemically strengthened glass and method for producing same

ABSTRACT

A chemically strengthened glass having a compressive stress layer formed in a surface layer thereof according to an ion exchange method, in which a surface of the glass has polishing flaws, the glass has a texture direction index (Stdi) of 0.30 or more, a hydrogen concentration Y in a region to a depth X from an outermost surface of the glass satisfies the following relational equation (I) at X=from 0.1 to 0.4 (μm), and a surface strength F (N) measured by a ball-on-ring test is (F≧1400×t 2 ) relative to a sheet thickness t (mm) of the glass: 
         Y=aX+b   (I)
 
     in which meanings of respective symbols in the equation (I) are as follows: Y: hydrogen concentration (as H 2 O, mol/L), X: depth from the outermost surface of the glass (μm), a: −0.300 or more, and b: 0.220 or less.

TECHNICAL FIELD

The present invention relates to a chemically strengthened glass and amethod for producing the same.

BACKGROUND ART

In flat panel display devices such as digital cameras, mobile phones,personal digital assistants (PDAs), etc., in order to protect displaysand enhance the appearance thereof, a thin plate-like cover glass isdisposed on the front surface of the display so as to provide a broaderregion than an image display portion. Although the glass has a hightheoretical strength, when scratched, its strength is largely lowered,and therefore, for the cover glass that is required to satisfy strength,a chemically strengthened glass having a compressive stress layer formedon the surface thereof through ion exchange or the like is used.

With the growing demand for weight reduction and thickness reduction inflat panel display devices, it is desired to thin cover glass itself.Accordingly, for satisfying the purpose, both the surfaces and the edgesurfaces of cover glass are desired to have further strength.

For increasing the strength of the chemically strengthened glass,heretofore, a surface etching treatment to be conducted after chemicalstrengthening treatment is known (Patent Document 1).

Here, regarding the strength of glass, it is known that the strength ofglass is lowered by the existence of hydrogen (water) in glass(Non-Patent Documents 1 and 2).

BACKGROUND ART DOCUMENT Patent Document

-   Patent Document 1: JP-T-2013-516387

Non-Patent Document

-   Non-Patent Document 1: S. ITO et. al., “Crack Blunting of    High-Silica Glass”, Journal of the American Ceramic Society, Vol.    65, No. 8, (1982), 368-371-   Non-Patent Document 2: Won-Taek Han et. al., “Effect of residual    water in silica glass on static fatigue”, Journal of Non-Crystalline    Solids, 127, (1991) 97-104

SUMMARY OF THE INVENTION Problems that the Invention is to Solve

The present inventors have found that there is a concern that thestrength of glass is lowered after the chemical strengthening, and themajor cause thereof is that moisture in the atmosphere penetrates intothe glass surface layer to form chemical defects. Further, the presentinventors have found that this phenomenon occurs not only throughchemical strengthening but also through a heating step in glassproduction process.

As a technique for removing moisture from a glass surface layer, it maybe considered to chip off the moisture-containing layer according to atechnique of polishing the glass surface after chemical strengthening oraccording to a technique of subjecting the glass surface after chemicalstrengthening to an etching treatment by immersing in hydrofluoric acidor the like. However, there is a concern that the surface of glass isscratched by polishing so that the strength thereof rather lowers. Inaddition, in a case where the glass surface has latent flaws, there is aconcern that the etching treatment using hydrofluoric acid or the likegrows the latent flaws to cause appearance failure owing to pits.Further, hydrofluoric acid requires careful handling in view of safety.

An object of the present invention is to provide a chemicallystrengthened glass capable of effectively preventing the strength ofglass from lowering even after performing chemical strengthening.

Means for Solving the Problems

The present inventors have found that by not only allowing a hydrogenconcentration profile on a surface layer of a chemically strengthenedglass to fall within a specific range but also controlling a texturedirection index (Stdi) of a glass surface to a specific value or more,even when the glass surface is polished, the surface strength of theglass is tremendously improved, thereby accomplishing the presentinvention.

Namely, the present invention is as shown below.

[1] A chemically strengthened glass having a compressive stress layerformed in a surface layer thereof according to an ion exchange method,

in which a surface of the glass has polishing flaws,

the glass has a texture direction index (Stdi) of 0.30 or more,

a hydrogen concentration Y in a region to a depth X from an outermostsurface of the glass satisfies the following relational equation (I) atX=from 0.1 to 0.4 (μm), and

a surface strength F (N) measured by a ball-on-ring test under thefollowing conditions is (F≧1400×t²) relative to a sheet thickness t (mm)of the glass:

Y=aX+b  (I)

in which meanings of respective symbols in the equation (1) are asfollows:

Y: hydrogen concentration (as H₂O, mol/L);

X: depth from the outermost surface of the glass (μm);

a: −0.300 or more; and

b: 0.220 or less,

the conditions of the ball-on-ring test:

a sheet of the glass having the sheet thickness t (mm) is disposed on astainless ring whose diameter is 30 mm and whose contact part has aroundness with a curvature radius of 2.5 mm; while a steel ball having adiameter of 10 mm is kept in contact with the sheet of the glass, acenter of the ring is subjected to a load by the ball under a staticloading condition; and a fracture load (unit: N) at which the glass isfractured is taken as a BOR surface strength and an average value oftwenty measured values thereof is taken as the surface strength F,provided that in a case where a fracture origin of the glass isseparated from a loading point of the ball by 2 mm or more, the obtainedvalue is excluded from data for calculating the average value.

[2] The chemically strengthened glass according to [1], in which theglass is an aluminosilicate glass, an aluminoborosilicate glass or asoda-lime glass.[3] The chemically strengthened glass according to [1] or [2], having atexture aspect ratio (Str20) of 0.10 or more.[4] A method for producing a chemically strengthened glass, including astep of bringing a glass containing sodium into contact with aninorganic salt containing potassium nitrate, thereby performing ionexchange of a Na ion in the glass with a K ion in the inorganic salt,

in which the inorganic salt contains at least one salt selected from thegroup consisting of K₂CO₃, Na₂CO₃, KHCO₃, NaHCO₃, K₃PO₄, Na₃PO₄, K₂SO₄,Na₂SO₄, KOH and NaOH, and

the method includes:

a step of polishing a surface of the glass before the ion exchange;

a step of washing the glass after the ion exchange;

a step of subjecting the glass to an acid treatment after the washing;and

a step of subjecting the glass to an alkali treatment after the acidtreatment.

[5] A chemically strengthened glass obtained by the production methodaccording to [4].

Advantage of the Invention

According to the chemically strengthened glass of the present invention,by allowing the hydrogen concentration profile on the glass surfacelayer to fall within a specific range and controlling the texturedirection index (Stdi) of the glass surface to a specific value or more,it is possible to significantly improve the surface strength of theglass.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic view for explaining a method of a ball-on-ringtest.

FIG. 2 A schematic view showing a production process of a chemicallystrengthened glass according to the present invention is shown in (a) to(d) of FIG. 2.

FIG. 3 is a graph of plotting the hydrogen concentration profile in thesurface layer of each chemically strengthened glass obtained in Examples1 to 2 and Comparative Examples 1 to 2.

FIG. 4 is an explanatory view for deriving the relational equation (I)from the graph of plotting the hydrogen concentration profile in thesurface layer of the chemically strengthened glass obtained in Example1.

FIG. 5 is an explanatory view for deriving the relational equation (I)from the graph of plotting the hydrogen concentration profile in thesurface layer of the chemically strengthened glass obtained inComparative Example 1.

FIG. 6 is an AFM image of a glass surface of Example 1.

FIG. 7 is an AFM image of a glass surface of Example 2.

FIG. 8 is an AFM image of a glass surface of Example 3.

FIG. 9 is an AFM image of a glass surface of Comparative Example 1.

FIG. 10 is an AFM image of a glass surface of Comparative Example 2.

FIG. 11 is an AFM image of a glass surface of Comparative Example 3.

FIG. 12 is an AFM image of a glass surface of Reference Example 1.

FIG. 13 is an AFM image of a glass surface of Reference Example 2.

FIG. 14 is a Weibull plot of BOR surface strength evaluation of each ofchemically strengthened glasses obtained in Example 1 and ComparativeExample 1.

MODE FOR CARRYING OUT THE INVENTION

The present invention is hereunder described in detail, but it shouldnot be construed that the present invention is limited to the followingembodiments, and the present invention may be arbitrarily modified andcarried out within the range where the gist of the present invention isnot deviated.

<Chemically Strengthened Glass>

The chemically strengthened glass according to the present invention isa chemically strengthened glass having a compressive stress layer formedin the surface layer thereof according to an ion exchange method, inwhich the hydrogen concentration in the region to a certain depth fromthe outermost surface of the glass satisfies the following relationalequation (I), and the glass surface has polishing flaws.

The compressive stress layer is a high-density layer formed through ionexchange of the Na ion in a glass surface with the K ion in a moltensalt by bringing a starting material glass into contact with aninorganic molten salt such as potassium nitrate.

In the chemically strengthened glass of the present invention, thehydrogen concentration profile in the glass surface layer falls within aspecific range. Specifically, the hydrogen concentration Y in a regionto a depth X from the outermost surface of the glass satisfies thefollowing relational equation (I) at X=from 0.1 to 0.4 (μm).

Y=aX+b  (I)

[In the equation (I), the meanings of the respective symbols are asfollows:

Y: hydrogen concentration (as H₂O, mol/L)

X: depth from the outermost surface of glass (μm)

a: −0.300 or more

b: 0.220 or less]

Regarding the surface strength of a glass, it is known that the surfacestrength of a glass lowers owing to the presence of hydrogen (moisture)in the glass, but the present inventors have found that there is aconcern that the surface strength of glass is lowered after the chemicalstrengthening treatment, and the major cause thereof is that moisture inthe atmosphere penetrates into the glass to form chemical defects.Further, the present inventors have found that this phenomenon occursnot only through the chemical strengthening but also through a heatingstep in glass production process.

When the hydrogen concentration in a glass is high, hydrogen penetratesinto the Si—O—Si bond network in the glass in the form of Si—OH wherebythe bond of Si—O—Si is cut. When the hydrogen concentration in the glassis high, it is considered that the part where the Si—O—Si bond is cutincreases so that chemical defects may be easily formed, whereby thesurface strength is lowered.

The above-mentioned relational equation (I) holds in a region of fromthe outermost surface to a depth X=from 0.1 to 0.4 The thickness of thecompressive stress layer to be formed through ion exchange falls withina range of from 5 to 50 μm, though it depends on the degree of chemicalstrengthening. The hydrogen penetration depth into glass follows thediffusion coefficient, temperature and time, and the hydrogenpenetration amount is influenced by the moisture amount in theatmosphere in addition to these. The hydrogen concentration afterchemical strengthening is the highest in the outermost layer andgradually reduces toward the deep part (bulk) where the compressivestress layer is not formed. The above-mentioned relational equation (I)defines the profile of the reduction, and in the outermost surface (X=0μm), there is a possibility that the moisture concentration may varyowing to time-dependent degradation, and therefore the equation isdefined to hold in a region of the near surface (X=from 0.1 to 0.4 μm)not influenced by the variation.

In the equation (I), a indicates an inclination to define the profile ofreduction in the hydrogen concentration. The range of a is −0.300 ormore, preferably from −0.270 to −0.005, more preferably from −0.240 to−0.030.

In the equation (I), b corresponds to the hydrogen concentration in theoutermost surface (X=0 μm). The range of b is 0.220 or less, preferablyfrom 0.020 to 0.215, more preferably from 0.030 to 0.210.

In general, the surface strength reduction of a glass is considered tobe caused by growth of microcracks existing in the glass surface owingto the mechanical stress given thereto from the outside. According toNon-Patent Document 2, when the glass structure at the tip of a crack isin a Si—OH-richer state, it is considered that the cracks easily grow.Assuming that the tips of cracks are exposed out in the atmosphere, theSi—OH amount in the tip of the crack is presumed to have a positiverelationship to the hydrogen concentration in the glass outermostsurface. Accordingly, b corresponding to the hydrogen concentration inthe outermost surface preferably falls within a low range to the degreeas shown above.

As shown in FIG. 3, the glass processed through a chemical strengtheningstep did not show any remarkable difference in the hydrogen penetrationdepth. There is a possibility that the hydrogen penetration depth mayvary depending on the condition of the chemical strengthening step, butif the depth does not change at all, there appears a negativecorrelation between b that corresponds to the hydrogen concentration inthe outermost surface and a that corresponds to the inclination todefine the profile of reduction in the hydrogen concentration.Accordingly, a preferably falls within a high range to a degree as shownabove.

As described above, in the present invention, it has been found that,not only by defining the hydrogen concentration itself alone in thesurface layer but also by defining the hydrogen concentration in thesurface layer and the reduction profile thereof each to fall within aspecific range, with taking particular note of the hydrogenconcentration profile, the surface strength of chemically strengthenedglass can be greatly improved.

[Method for Measuring Hydrogen Concentration Profile]

Here, the hydrogen concentration profile (H₂O concentration, mol/L) of aglass is a profile measured under the following analysis condition.

For measurement of the hydrogen concentration profile of a glasssubstrate, a method of secondary ion mass spectrometry (SIMS) isemployed. In a case where a quantitative hydrogen concentration profileis obtained through SIMS, a standard sample whose hydrogen concentrationis known is necessary. A method for preparing the standard sample and amethod for determination of the hydrogen concentration thereof aredescribed below.

1) A part of the glass substrate to be analyzed is cut out.2) A region of 50 μm or more from the surface of the thus-cut glasssubstrate is removed by polishing or chemical etching. The removaltreatment is carried out on both surfaces. Namely, the thickness to beremoved on both surfaces is 100 μm or more. The glass substrate that hasbeen subjected to the removal treatment is used as a standard sample.3) The standard sample is analyzed through infrared spectroscopy (IR),and the absorbance height A₃₅₅₀ at the peak top near 3,550 cm⁻¹ in theIR spectrum and the absorbance height A₄₀₀₀ (base line) at 4,000 cm⁻¹are determined.4) The thickness d (cm) of the standard sample is measured using athickness measuring device such as a micrometer.5) With reference to the reference A, the hydrogen concentration (asH₂O, mol/L) in the standard sample is determined using the formula II,in which the infrared practical absorbance index of H₂O in glassε_(pract) (L/(mol·cm)) is 75.

Hydrogen concentration in standard sample=(A ₃₅₅₀ −A ₄₀₀₀)/(ε_(pract)·d)   Formula II

Reference A): S. Ilievski et al., Glastech. Ber. Glass Sci. Technol., 73(2000) 39.

The glass substrate to be analyzed and the standard sample whosehydrogen concentration is known, as prepared according to theabove-mentioned method, are simultaneously fed into a SIMS device, andanalyzed sequentially to obtain the depth direction profiles of theintensities of ¹H⁻ and ³⁰Si⁻. Subsequently, the ¹H⁻ profile is dividedby the ³⁰Si⁻ profile to obtain a depth direction profile of ¹H⁻/³⁰Si⁻intensity ratio. From the depth direction profile of the ¹H⁻/³⁰Si⁻intensity ratio of the standard sample, an average ¹H⁻/³⁰Si⁻ intensityratio in a region of a depth of from 1 μm to 2 μm is calculated, and acalibration curve of this value and the hydrogen concentration is drawnto pass through the origin (calibration curve with one-level standardsample). Using the calibration curve, the ¹H⁻/³⁰Si⁻ intensity ratio onthe vertical axis of the profile of the glass substrate to be analyzedis converted into the hydrogen concentration. Accordingly, the hydrogenconcentration profile of the glass substrate to be analyzed is obtained.The measurement conditions in SIMS and IR are as follows.

[SIMS Measurement Condition]

Device: ADEPT1010 manufactured by ULVAC-PHI, Inc.,Primary ion species: Cs⁺Primary ion accelerating voltage: 5 kVPrimary ion current value: 500 nAPrimary ion incident angle: 60° relative to the normal line of thesample planePrimary ion luster size: 300×300 μm²Secondary ion polarity: minusSecondary ion detection region: 60×60 μm² (4% of luster size of primaryion)

ESA Input Lens: 0

Use of neutralization gun: yesMethod of converting the horizontal axis from sputtering time to depth:The depth of the analysis crater is measured with a stylus surfaceprofile analyzer (Dektak 150, manufactured by Veeco Inc.), and theprimary ion sputtering rate is determined. Using the sputtering rate,the horizontal axis is converted from the sputtering time to the depth.Field Axis Potential in ¹H⁻ detection: The optimum value may change inevery device. The operator should carefully define the value so that thebackground is fully cut off.

[IR Measurement Condition]

Device: Nic-plan/Nicolet 6700, manufactured by Thermo Fisher ScientificCo., Ltd.Resolution: 4 cm⁻¹Number of scans: 16Detector: TGS detector

For deriving the relational equation (I) from the hydrogen concentrationprofile (H₂O concentration, mol/L) of the glass determined under theabove-mentioned analysis condition, the following procedure is employed.As shown in FIG. 4 and FIG. 5, linear approximation is applied to thehydrogen concentration profile in a region of a depth of from 0.1 to 0.4μm. The equation of the resultant approximation straight line isreferred to as the relational equation (I).

As a means of controlling a and b, for example, the fusing agentconcentration, sodium concentration, temperature and time in thechemical strengthening step may be changed.

The chemically strengthened glass of the present invention has polishingflaws on the surface thereof. Here, polishing in the present inventionmeans that the surface of a glass is polished with abrasives forsmoothing. A method of surface polishing is not particularly limited,and examples thereof include usual methods. The presence or absence ofpolishing flaws may be discerned through surface observation with AFM(Atomic Force Microscope). A case where one or more scratches having alength of 5 μm or more are present in a region of 10 μm×5 μm can be saidto be in a state that the surface has polishing flaws. FIG. 6 shows astate having surface polishing flaws (in Example 1 to be given below),and FIG. 12 shows a state not having surface polishing flaws (inReference Example 1 to be given below).

(Glass Surface Strength)

The surface strength of the chemically strengthened glass of the presentinvention can be evaluated according to a ball-on-ring test.

(Ball-on-Ring Test)

The chemically strengthened glass of the present invention is evaluatedin terms of the BOR surface strength F (N) measured by a ball-on-ring(BOR) test, in which a glass sheet is disposed on a stainless ring whosediameter is 30 mm and whose contact part has a roundness with acurvature radius of 2.5 mm, and while a steel ball having a diameter of10 mm is kept in contact with the glass sheet, the center of the ring issubjected to a load by the ball under a static loading condition.

The chemically strengthened glass of the present invention satisfiesF≧1,400×t², preferably F≧1,800×t². [In the formulae, F means the BORsurface strength (N) measured by the ball-on-ring test, and t means thethickness (mm) of the glass sheet.] When the BOR surface strength F (N)falls within the range, the glass exhibits an excellent surface strengtheven when formed into a thin sheet.

FIG. 1 shows a schematic view for explaining the ball-on-ring test usedin the present invention. In the ball-on-ring (BOR) test, a glass sheet1 is, while kept set horizontally, pressurized by a pressurizing jig 2made of SUS304 (hardened steel, diameter: 10 mm, mirror-finished) tomeasure the surface strength of the glass sheet 1.

In FIG. 1, the glass sheet 1 to be a sample is horizontally set on areceiving jig 3 made of SUS304 (diameter: 30 mm, radius of curvature ofthe contact part R: 2.5 mm, the contact part is hardened steel,mirror-finished). Above the glass sheet 1, a pressurizing jig 2 forpressurizing the glass sheet 1 is arranged.

In this embodiment, the center region of the glass sheet 1 obtained inExamples and Comparative Examples is pressurized from above. The testcondition is as mentioned below.

Descending Rate of Pressurizing Jig 2: 1.0 (mm/min)

In this test, the fracture load (unit: N) at which the glass isfractured is taken as a BOR surface strength. The average value oftwenty measured values thereof is taken as a surface strength F.However, in a case where the fracture origin of the glass sheet isseparated from a loading point of the ball by 2 mm or more, the obtainedvalue is excluded from the data for calculating the average value.

(Texture Direction Index (Stdi))

Furthermore, in the chemically strengthened glass of the presentinvention, a texture direction index (Stdi) thereof is 0.30 or more,preferably 0.45 or more, and more preferably 0.50 or more. The texturedirection index (Stdi) as referred to herein is an index exhibitingsuperiority or inferiority of directionality of the texture formed onthe surface and takes a value of from 0 to 1. In the case where thetexture has a superior direction, the Stdi becomes close to 0. On theother hand, in the case where the texture does not have directionality,the Stdi becomes close to 1. That is, in the case where the texture hasa superior direction as in polishing flaws, the Stdi takes a value closeto 0. However, when the polishing flaws become smooth, and the texturebecomes indistinct, it may be considered that the Stdi becomes closeto 1. It may be considered that in view of the fact that the texturedirection index (Stdi) of the chemically strengthened glass of thepresent invention falls within the foregoing range, the chemicallystrengthened glass of the present invention has smooth polishing flaws.

After obtaining a shape image by an atomic force microscope (AFM), thetexture direction index (Stdi) may be determined by an image analysissoftware.

(Texture Aspect Ratio (Str20))

The chemically strengthened glass of the present invention alsopreferably has a texture aspect ratio (Str20) of 0.10 or more, morepreferably 0.35 or more, and still more preferably 0.55 or more. It maybe considered that when the texture aspect ratio (Str20) falls withinthe foregoing range, the chemically strengthened glass of the presentinvention has smooth polishing flaws.

After obtaining a shape image by an atomic force microscope (AFM), thetexture aspect ratio (Str20) may be determined by an image analysissoftware.

In addition, the chemically strengthened glass of the present inventionis less in light scattering to be caused due to a pit and is excellentin surface appearance.

<Method for Producing Chemically Strengthened Glass>

One embodiment of the method for producing a chemically strengthenedglass of the present invention is described below, to which, however,the present invention is not limited.

(Glass Composition)

Glass for use in the present invention may be any one containing sodium,and so far as it has a composition capable of being shaped andstrengthened through chemical strengthening treatment, various types ofcompositions can be used. Specifically, for example, there are mentionedan aluminosilicate glass, a soda-lime glass, a boron silicate glass, alead glass, an alkali barium glass, an aluminoborosilicate glass, etc.

The production method for a glass is not specifically limited. Desiredglass raw materials are put into a continuous melting furnace, and theglass raw materials are melted under heat at preferably from 1,500 to1,600° C., then refined and fed into a shaping device to shape themolten glass into a plate-like shape and gradually cooled to produce aglass.

Various methods may be employed for shaping a glass. For example,various shaping processes such as a down-draw process (for example, anoverflow down-draw process, a slot-down process, a redraw process,etc.), a float process, a roll-out process, and a pressing process maybe employed.

The thickness of a glass is not specifically limited, but foreffectively conducting chemical strengthening treatment, in general, thethickness thereof is preferably 5 mm or less, more preferably 3 mm orless.

The shape of a glass for use in the present invention is notspecifically limited. For example, various shapes of a glass such as aplate-like shape having a uniform thickness, a curved shape in which atleast one of the front surface or the back surface is curved, and athree-dimensional shape having a bend portion are employable.

Although the composition of the chemically strengthened glass of thepresent invention is not specifically limited, for example, thefollowing glass compositions may be mentioned.

(i) Glass having a composition including, in terms of mol %, from 50 to80% of SiO₂, from 2 to 25% of Al₂O₃, from 0 to 10% of Li₂O, from 0 to18% of Na₂O, from 0 to 10% of K₂O, from 0 to 15% of MgO, from 0 to 5% ofCaO and from 0 to 5% of ZrO₂.

(ii) Glass having a composition including, in terms of mol %, from 50 to74% of SiO₂, from 1 to 10% of Al₂O₃, from 6 to 14% of Na₂O, from 3 to11% of K₂O, from 2 to 15% of MgO, from 0 to 6% of CaO and from 0 to 5%of ZrO₂, in which the total content of SiO₂ and Al₂O₃ is 75% or less,the total content of Na₂O and K₂O is from 12 to 25%, and the totalcontent of MgO and CaO is from 7 to 15%.

(iii) Glass having a composition including, in terms of mol %, from 68to 80% of SiO₂, from 4 to 10% of Al₂O₃, from 5 to 15% of Na₂O, from 0 to1% of K₂O, from 4 to 15% of MgO and from 0 to 1% of ZrO₂.

(iv) Glass having a composition including, in terms of mol %, from 67 to75% of SiO₂, from 0 to 4% of Al₂O₃, from 7 to 15% of Na₂O, from 1 to 9%of K₂O, from 6 to 14% of MgO and from 0 to 1.5% of ZrO₂, in which thetotal content of SiO₂ and Al₂O₃ is from 71 to 75%, the total content ofNa₂O and K₂O is from 12 to 20%, and the content of CaO, if any, is lessthan 1%.

The chemically strengthened glass of the present invention has anion-exchanged compressive stress layer in the surface thereof. In theion exchange method, the surface of a glass is ion-exchanged to form asurface layer in which compressive stress remains. Specifically, thealkali metal ion (typically Li ion, Na ion) having a small ionic radiusin the surface of a glass sheet is substituted with an alkali ion havinga larger ionic radius (typically Na ion or K ion for Li ion, and K ionfor Na ion) through ion exchange at a temperature not higher than theglass transition point. Accordingly, compressive stress remains in thesurface of the glass, and the surface strength of the glass is therebyincreased.

In the production method of the present invention, chemicalstrengthening is conducted by bringing a glass into contact with aninorganic salt containing potassium nitrate (KNO₃). Accordingly, the Naion in the glass surface is ion-exchanged with the K ion in theinorganic salt to form a high-density compressive stress layer. Themethod for bringing a glass into contact with an inorganic salt includesa method of applying a pasty inorganic salt to a glass, a method ofspraying a glass with an aqueous solution of an inorganic salt, and amethod of immersing a glass in a salt bath of a molten salt heated at atemperature not lower than the melting point thereof, and of these, amethod of immersing in a molten salt is desirable.

The inorganic salt is preferably one having a melting point not higherthan the strain point of the glass to be strengthened (generally 500 to600° C.), and in the present invention, a salt containing potassiumnitrate (melting point: 330° C.) is used. Containing potassium nitrate,the salt is capable of being in a molten state at a temperature nothigher than the strain point of the glass and, in addition, capable ofbeing easily handled in the operating temperature range. The content ofthe potassium nitrate in the inorganic salt is preferably 50% by mass ormore.

Additionally, the inorganic salt contains at least one salt selectedfrom the group consisting of K₂CO₃, Na₂CO₃, KHCO₃, NaHCO₃, K₃PO₄,Na₃PO₄, K₂SO₄, Na₂SO₄, KOH and NaOH, and above all, preferably containsat least one salt selected from the group consisting of K₂CO₃, Na₂CO₃,KHCO₃ and NaHCO₃.

The above-mentioned salt (hereinafter this may be referred to as “fusingagent”) has a property of cutting the network of a glass typified by anSi—O—Si bond. Since the temperature at which chemical strengtheningtreatment is conducted is high such as a few hundred degrees C., thecovalent bond between Si—O in a glass is suitably cut at thattemperature and therefore the density-reducing treatment to be mentionedbelow for the glass can be easy to promote.

The degree of cutting the covalent bond may vary depending on the glasscomposition, the type of the salt (fusing agent) to be used, and thechemical strengthening treatment conditions such as the temperature andthe time, but is considered to be preferably selected from theconditions under which from 1 to 2 bonds of the four covalent bondsextending from Si can be cut.

For example, in a case where K₂CO₃ is used as a fusing agent, thecontent of the fusing agent in the inorganic salt is 0.1 mol % or moreand the chemical strengthening treatment temperature is from 350 to 500°C., the chemical strengthening treatment time is preferably from 1minute to 10 hours, more preferably from 5 minutes to 8 hours, even morepreferably from 10 minutes to 4 hours.

The amount of the fusing agent to be added is, from the viewpoint ofsurface hydrogen concentration control, preferably 0.1 mol % or more,more preferably 1 mol % or more, and particularly preferably 2 mol % ormore. From the viewpoint of productivity, the amount thereof ispreferably not larger than the saturation solubility of each salt. Whenthe fusing agent is excessively added, there is a concern of causingglass corrosion. For example, in a case where K₂CO₃ is used as thefusing agent, the amount thereof is preferably 24 mol % or less, morepreferably 12 mol % or less, particularly preferably 8 mol % or less.

The inorganic salt may contain any other chemical species within a rangenot impairing the advantageous effects of the present invention, inaddition to potassium nitrate and the fusing agent. For example, thereare mentioned alkali chloride salts or alkali borate salts such assodium chloride, potassium chloride, sodium borate, and potassiumborate. One or more of these may be added either singly or as combined.

(Step 1: Polishing of Glass Surface)

In the step 1, the glass surface is polished. The polishing conditionsare not particularly limited, and the polishing may be performed underconditions so as to obtain a desired surface roughness. As for polishingmeans, the polishing may be performed by a general method in which, forexample, cerium oxide having an average particle diameter of about 0.7μm is dispersed in water to prepare a slurry having a specific gravityof 0.9, and the glass surface is polished in a depth of 0.5 μm or moreper surface with a polishing pad of a nonwoven type or suede type underconditions of a polishing pressure of 10 kPa.

By polishing the glass surface, macro flaws on the glass surface areremoved, whereby the surface strength can be improved. The polishing ofthe glass surface is performed before a chemical strengthening treatment(ion exchange) of a step 4 as described later.

The production method of the present invention is hereunder describedwith reference to examples of an embodiment in which chemicalstrengthening is performed according to a method of immersing a glass ina molten salt.

(Production of Molten Salt 1)

A molten salt may be produced according to steps mentioned below.

Step 2a: Preparation of potassium nitrate molten saltStep 3a: Addition of fusing agent to the potassium nitrate molten salt(Step 2a—Preparation of Potassium Nitrate Molten Salt—)

In the step 2a, potassium nitrate is put into a container, and melted byheating at a temperature not lower than the melting point thereof toprepare a molten salt. The melting is conducted at a temperature fallingwithin a range of from the melting point (330° C.) of potassium nitrateto the boiling point (500° C.) thereof. In particular, it is morepreferable that the melting temperature is from 350 to 470° C. from theviewpoint of the balance between the surface compressive stress (CS) tobe given to a glass and the depth of the compressive stress layer (DOL)and of the strengthening time.

Regarding the container for melting potassium nitrate, metals, quartz,ceramics and the like can be used. Above all, from the viewpoint ofdurability, metal materials are desirable, and from the viewpoint ofcorrosion resistance, stainless steel (SUS) materials are preferred.

(Step 3a—Addition of Fusing Agent to the Potassium Nitrate Molten Salt—)

In the step 3a, the above-mentioned fusing agent is added to thepotassium nitrate molten salt prepared in the step 2a, and, while keptat a temperature falling within a certain definite range, mixed with animpeller or the like so that the whole becomes uniform. In a case whereplural fusing agents are used, the order of adding them is notspecifically limited, and these may be added at a time.

The temperature is preferably not lower than the melting point ofpotassium nitrate, that is, preferably 330° C. or higher, morepreferably from 350 to 500° C. The stirring time is preferably from 1minute to 10 hours, more preferably from 10 minutes to 2 hours.

(Production of Molten Salt 2)

In the above-mentioned production of molten salt 1, a method of adding afusing agent after preparation of a molten salt of potassium nitrate isexemplified, but apart from the method, the molten salt may also beproduced according to the following steps.

Step 2b: mixing of potassium nitrate and fusing agentStep 3b: melting of mixed salt of potassium nitrate and fusing agent(Step 2b—Mixing of Potassium Nitrate and Fusing Agent—)

In the step 2b, potassium nitrate and a fusing agent are put into acontainer and mixed with an impeller or the like. In a case where pluralfusing agents are used, the order of adding them is not specificallylimited, and these may be added at a time. The container to be used maybe the same one as that to be used in the above-mentioned step 2a.

(Step 3b—Melting of Mixed Salt of Potassium Nitrate and Fusing Agent—)

In the step 3b, the mixed salt obtained in the step 2b is melted byheating. The melting is conducted at a temperature falling within arange of from the melting point (330° C.) of potassium nitrate to theboiling point (500° C.) thereof. In particular, it is more preferablethat the melting temperature is from 350 to 470° C. from the viewpointof the balance between the surface compressive stress (CS) to be givento a glass and the depth of the compressive stress layer (DOL) and ofthe strengthening time. The stirring time is preferably from 1 minute to10 hours, more preferably from 10 minutes to 2 hours.

In a case where sediments form in the molten salt obtained through theabove-mentioned step 2a and the step 3a, or through the step 2b and thestep 3b, by adding a fusing agent thereto, the molten salt is keptstatically until the sediments precipitate in the bottom of thecontainer, before the chemical strengthening treatment for a glass. Thesediments contain the fusing agent exceeding the saturation solubilitythereof, and salts formed through exchange of cation in the fusing agentin the molten salt.

The molten salt for use in the production method of the presentinvention has an Na concentration of preferably 500 ppm by weight ormore, more preferably 1,000 ppm by weight or more. The Na concentrationof 500 ppm by weight or more in the molten salt is preferred since thelow-density layer can easily deepen in the acid treatment step to bementioned hereinunder. The upper limit of the Na concentration is notspecifically defined, and is acceptable to a level at which a desiredsurface compressive stress (CS) can be obtained.

The molten salt used for chemical strengthening treatment once or morecontains sodium released from a glass. Therefore, when the Naconcentration is already within the above-mentioned range, glass-derivedsodium may be used as such for the Na source, or when the Naconcentration is insufficient or when a fresh molten salt that has notbeen used for chemical strengthening treatment is used, the Naconcentration may be controlled by adding an inorganic sodium salt suchas sodium nitrate.

As described above, a molten salt can be prepared according to theabove-mentioned step 2a and the step 3a, or the step 2b and the step 3b.

(Chemical Strengthening)

Next, using the prepared molten salt, chemical strengthening treatmentis performed. In the chemical strengthening treatment, a glass isimmersed in a molten salt and the metal ion (Na ion) in the glass issubstituted with a metal ion (K ion) having a larger ionic radius in themolten salt. Through the ion exchange, the composition of the glasssurface is changed, and the glass surface is densified to form acompressive stress layer 20 [(a) to (b) in FIG. 2]. The densification ofthe glass surface generates compressive stress to strengthen the glass.

In fact, the density of chemically strengthened glass graduallyincreases from the outer edge of the interlayer 30 (bulk) existing inthe center of the glass toward the surface of the compressive stresslayer, and therefore between the interlayer 30 and the compressivestress layer 20, there exists no definite boundary at which the densitysuddenly changes. Here, the interlayer means a layer existing in thecenter part of the glass and surrounded by the compressive stress layer.The interlayer is a layer not undergone ion exchange, differing from thecompressive stress layer.

Specifically, the chemical strengthening treatment in the presentinvention is performed by the following step 4.

Step 4: Chemical Strengthening Treatment for Glass

In the step 4, a glass is preheated, and the temperature of the moltensalt prepared in the above-mentioned step 2a and the step 3a or the step2b and the step 3b is adjusted to a temperature for chemicalstrengthening. Next, the preheated glass is immersed in the molten saltfor a predetermined period of time, then the glass is drawn up from themolten salt and left cooled. Preferably, prior to the chemicalstrengthening treatment, the glass is processed for shaping inaccordance with the use thereof, for example, through mechanicalprocessing such as cutting, end fsurace machining, drilling, etc.

The glass preheating temperature depends on the temperature at which theglass is immersed in a molten salt, but, in general, preferably 100° C.or higher.

The chemical strengthening temperature is preferably not higher than thestrain point of the glass to be strengthened (generally 500 to 600° C.),but for obtaining a greater compressive stress layer depth, particularlypreferably 350° C. or higher.

The immersion time for the glass in a molten salt is preferably from 1minute to 10 hours, more preferably from 5 minutes to 8 hours, even morepreferably from 10 minutes to 4 hours. Falling within the range, it ispossible to obtain a chemically strengthened glass excellent in thebalance between the surface strength and the depth of the compressivestress layer.

In the production method of the present invention, the following stepsare performed after the chemical strengthening treatment.

Step 5: Washing of the Glass

Step 6: Acid Treatment of the Glass after Step 5

At the time after the above-mentioned step 6, the glass surface furtherhas a low-density layer 10 in which the surface layer of the compressivestress layer has been denatured, specifically, the density thereof hasbeen reduced [(b) to (c) in FIG. 2]. The low-density layer is formedthrough leaching of Na and K from the outermost surface of thecompressive stress layer, and in place of these, H has penetrated(substituted) therein.

The step 5 and the step 6 are described in detail hereinunder.

(Step 5—Washing of Glass—)

In the step 5, the glass is washed with industrial water, ion-exchangedwater or the like. Above all, ion-exchanged water is preferred. Thewashing condition may vary depending on the washing liquid to be used,but in a case where ion-exchanged water is used, it is preferable thatthe glass is washed at 0 to 100° C. from the viewpoint of completelyremoving the adhered salts.

(Step 6—Acid Treatment—)

In the step 6, the glass washed in the step 5 is further subjected to anacid treatment.

In the acid treatment for a glass, a chemically strengthened glass isimmersed in an acidic solution, whereby Na and/or K in the surface ofthe chemically strengthened glass can be substituted with H.

The solution is not specifically limited so far as it is acidic and hasa pH of less than 7, in which the acid to be used may be a weak acid ora strong acid. Specifically, the acid is preferably hydrochloric acid,sulfuric acid, phosphoric acid, acetic acid, oxalic acid, carbonic acid,citric acid, etc. These acids may be used either singly or as combined.

The temperature for performing the acid treatment varies depending onthe type and the concentration of the acid to be used and the treatingtime, but is preferably 100° C. or lower.

The time for performing the acid treatment also varies depending on thetype, the concentration and the temperature of the acid to be used, butis preferably from 10 seconds to 5 hours from the viewpoint ofproductivity, more preferably from 1 minute to 2 hours.

The concentration of the solution for performing the acid treatmentvaries depending on the type and the temperature of the acid to be usedand the treating time, but is preferably a concentration in which riskof container corrosion is less, and specifically, the concentrationthereof is preferably from 0.1 wt % to 20 wt %.

The low-density layer is removed in alkali treatment to be mentionedbelow, and a thicker low-density layer is preferred as the glass surfaceis easy to remove. Accordingly, the thickness of the low-density layeris preferably 5 nm or more from the viewpoint of the amount of glasssurface removal, more preferably 20 nm or more. The thickness of thelow-density layer may be controlled by controlling the fusing agentconcentration, the sodium concentration, the temperature, the time andthe like in the chemical strengthening step.

The density of the low-density layer is preferably lower than thedensity in the region (bulk) deeper than the ion-exchanged compressivestress layer, from the viewpoint the glass surface removability.

The thickness of the low-density layer may be determined from the period(Δθ) measured in X-ray reflectometry (XRR).

The density of the low-density layer may be determined from the criticalangle (θc) measured in XRR.

In a simplified manner, formation of a low-density layer and thethickness of the layer may be confirmed through observation of the crosssection of a glass with a scanning electronic microscope (SEM).

In the production method of the present invention, the following step isperformed after the acid treatment.

Step 7: Alkali Treatment

In the step 7, a part or all of the low-density layer formed up to thestep 6 may be removed [(c) to (d) in FIG. 2].

The step 7 is described in detail hereinunder.

(Step 7—Alkali Treatment—)

In the step 7, the glass having been subjected to the acid treatment inthe step 6 is further subjected to an alkali treatment.

In the alkali treatment, the chemically strengthened glass is immersedin a basic solution, whereby a part or all of the low-density layer isremoved.

The solution is not specifically limited so far as it is basic and has apH of more than 7, in which any of a weak base or a strong base isusable. Specifically, a base such as sodium hydroxide, potassiumhydroxide, potassium carbonate, sodium carbonate or the like ispreferred. These bases may be used either singly or as combined.

The temperature for performing the alkali treatment varies depending onthe type and the concentration of the base to be used and the treatingtime, but is preferably from 0 to 100° C., more preferably from 10 to80° C., even more preferably from 20 to 60° C. The temperature range ispreferred as causing no risk of glass corrosion.

The time for performing the alkali treatment also varies depending onthe type, the concentration and the temperature of the base to be used,but is preferably from 10 seconds to 5 hours from the viewpoint ofproductivity, more preferably from 1 minute to 2 hours.

The concentration of the solution for performing the alkali treatmentvaries depending on the type and the temperature of the base to be usedand the treating time, but is preferably from 0.1 wt % to 20 wt % fromthe viewpoint of glass surface removability.

Through the above-mentioned alkali treatment, a part or all of thelow-density layer with H having penetrated thereinto is removed and thesurface layer in which the hydrogen concentration profile satisfies thespecific relational equation (I) described above is exposed out.Accordingly, a chemically strengthened glass having an improved surfacestrength can be obtained. Further, since the low-density layer isremoved, the flaws existing in the glass surface are also removed at thesame time. Therefore, it is considered that this point also contributesto the strength improvement.

In addition, in the production method of the present invention, bypolishing the glass surface before the chemical strengthening treatment,large surface flaws are removed in advance. It may be conjectured thataccording to this pre-polishing, the chemical strengthening treatment,and the acid and alkali treatments, macro flaws and micro flaws on theglass surface are removed, whereby the surface strength of the glass isremarkably improved.

Between the above-mentioned acid treatment step 6 and the alkalitreatment step 7, or after the alkali treatment step 7, it is preferableto perform a washing step like the step 5.

In the production method of the present invention, the chemical liquidsto be handled are highly safe and therefore the method does not requireany special equipment. Accordingly, a chemically strengthened glasswhose surface strength has dramatically increased can be obtained safelyand efficiently.

The amount of the low-density layer to be removed depends on the alkalitreatment condition. An embodiment in which the low-density layer 10 hasbeen completely removed is shown in (d) of FIG. 2, however, a part ofthe low-density layer 10 may be removed while a part thereof hasremained. From the viewpoint of strength improvement, the effect can beobtained even when not all the low-density layer is removed, but fromthe viewpoint of stably securing the transmittance of glass, it ispreferable that all the low-density layer is removed.

According to the method for producing a chemically strengthened glass ofthe present invention, even when polished, a chemically strengthenedglass with an improved surface strength can be obtained. Then, since thetreatment can be allowed to proceed by immersing in the solution, theproduction method is efficient from the standpoints that it is liable tocope with various glass shapes and large-area glasses; and that the bothsurfaces of the glass can be simultaneously treated. In addition, evenin the case where latent flaws are present in advance on the glasssurface, a chemically strengthened glass free from an appearance failuredue to a pit and with an improved surface strength can be obtained.Furthermore, as compared with etching treatment using hydrofluoric acidor the like, the treatment in the present invention is highly safe andinexpensive.

EXAMPLES

The present invention is described specifically with reference toExamples given below, but the present invention is not limited thereto.

<Evaluation Method>

Various evaluations in present Examples were performed according to theanalysis methods mentioned below.

(Evaluation of Glass: Surface Stress)

The compressive stress value of the compressive stress layer and thedepth of the compressive stress layer in the chemically strengthenedglass of the present invention can be measured using EPMA (electronprobe microanalyzer) or a surface stress meter (for example, FSM-6000manufactured by Orihara Manufacturing Co., Ltd.), etc. In Examples, thesurface compressive stress value (CS, unit: MPa) and the depth of thecompressive stress layer (DOL, unit: μm) were measured using a surfacestress meter (FSM-6000) manufactured by Orihara Manufacturing Co., Ltd.

(Evaluation of Glass: Removal Amount)

The removal amount thickness of a glass was determined by measuring theweight thereof before and after chemical liquid treatment, using ananalytical electronic balance (HR-202i, manufactured by A&D Company,Limited), and converting the found value into a thickness according tothe following equation.

(Removal amount thickness per one surface)=((weight beforetreatment)−(weight after treatment))/(glass specific gravity)/treatedarea/2

At this time, the calculation was made while defining the glass specificgravity as 2.41 (g/cm³).

(Evaluation of Glass: Texture Direction Index (Stdi))

First of all, a shape image was obtained by using an atomic forcemicroscope (XE-HDM, manufactured by Park Systems Corporation) underconditions of measurement mode: non-contact mode, scanning size: 10 μm×5μm, color scale: ±1 nm, scanning speed: 1 Hz, cantilever: Non-contactCantilever (Item: PPP-NCHR 10M, manufactured by Park SystemsCorporation). Thereafter, a leveling treatment and an L-filteringtreatment (ISO value: 2.0 μm) of the shape image were performed by usingan image analysis software (SPIP 6.2.6, manufactured by Image MetrologyInc.), and a texture direction index (Stdi) was determined by aroughness analysis.

(Evaluation of Glass: Texture Aspect Ratio (Str20))

First of all, a shape image was obtained by using an atomic forcemicroscope (XE-HDM, manufactured by Park Systems Corporation) underconditions of measurement mode: non-contact mode, scanning size: 10 μm×5μm, color scale: ±1 nm, scanning speed: 1 Hz, cantilever: Non-contactCantilever (Item: PPP-NCHR 10M, manufactured by Park SystemsCorporation). Thereafter, a leveling treatment and an L-filteringtreatment (ISO value: 2.0 μm) of the shape image were performed by usingan image analysis software (SPIP 6.2.6, manufactured by Image MetrologyInc.), and a texture aspect index (Str20) was determined by a roughnessanalysis.

(Evaluation of Glass: Surface Strength)

The glass surface strength was measured according to the ball-on-ring(BOR) test. FIG. 1 shows a schematic view for explaining theball-on-ring test employed in the present invention. A glass sheet 1was, while kept set horizontally, pressurized by a pressurizing jig 2made of SUS304 (hardened steel, diameter 10 mm, mirror-finished) tomeasure the surface strength of the glass sheet 1.

In FIG. 1, the glass sheet 1 to be a sample is horizontally disposed ona receiving jig 3 made of SUS304 (diameter: 30 mm, curvature radius ofthe contact part R: 2.5 mm, the contact part is hardened steel,mirror-finished). Above the glass sheet 1, a pressurizing jig 2 forpressurizing the glass sheet 1 is arranged.

In this embodiment, the center region of the glass sheet 1 obtained inExamples and Comparative Examples was pressurized from the above of theglass sheet 1. The test condition is as mentioned below.

Descending Rate of Pressurizing Jig 2: 1.0 (mm/min)

In this test, the fracture load (unit: N) at which the glass wasfractured was taken as a BOR surface strength. The average value oftwenty measured values thereof was taken as a surface strength F.However, in a case where the fracture origin of the glass sheet wasseparated from a loading point of the ball by 2 mm or more, the obtainedvalue was excluded from the data for calculating the average value.

(Evaluation of Glass: Hydrogen Concentration)

According to the method described in the section of [Method forMeasuring Hydrogen Concentration Profile] given hereinabove, thehydrogen concentration profile was determined and the relationalequation (I) was derived therefrom.

Example 1 Polishing Step

An aluminosilicate glass having dimensions of 50 mm×50 mm×0.70 mm wasprepared, cerium oxide having an average particle diameter of 1.2 μm wasdispersed in water to prepare a slurry having a specific gravity of 0.9,and the both surfaces of the aluminosilicate glass were simultaneouslypolished in a depth of 3.0 μm per surface with a polishing pad (nonwoventype) under conditions of a polishing pressure of 10 kPa. A surfaceroughness (Ra) as measured by the AFM measurement was 0.45 nm.

AFM measurement conditions: Atomic force microscope (XE-HDM,manufactured by Park Systems Corporation), scanning size: 10 μm×5 μm,color scale: ±1 nm, scanning speed: 1 Hz

(Chemical Strengthening Step)

In an SUS-made cup, 9,700 g of potassium nitrate, 890 g of potassiumcarbonate, and 400 g of sodium nitrate were introduced, and the contentswere heated to 450° C. by a mantle heater to prepare a molten saltcontaining 6 mol % of potassium carbonate and 10,000 ppm by weight ofsodium. The glass obtained in the polishing step was preheated to 200 to400° C. and then subjected to a chemical strengthening treatment byimmersing in the molten salt at 450° C. for 2 hours for ion exchange andcooling to around room temperature. The resultant chemicallystrengthened glass was washed with water and subjected to the next step.

Composition (in terms of mol %) of the aluminosilicate glass (specificgravity: 2.41): SiO₂ 68%, Al₂O₃ 10%, Na₂O 14%, MgO 8%

(Acid Treatment Step)

In a resin-made tank, 6.0 wt % of nitric acid (HNO₃, manufactured byKanto Chemical Co., Inc.) was prepared and subjected to temperatureadjustment to 41° C. by using a fluorine resin-covered heater (KKS14A,manufactured by Hakko Electric Co., Ltd.). The glass obtained in theabove-described chemical strengthening step was immersed in the nitricacid whose temperature was adjusted, for 120 seconds to perform the acidtreatment, followed by washing with pure water several times. Theresultant glass was subjected to the next step.

(Alkali Treatment Step)

An aqueous solution of 4.0 wt % sodium hydroxide was prepared in aresin-made tank and subjected to temperature adjustment to 40° C. byusing a fluorine resin-covered heater (KKS14A, manufactured by HakkoElectric Co., Ltd.). The glass obtained in the acid treatment step wasimmersed in the sodium hydroxide aqueous solution whose temperature wasadjusted, for 120 seconds to perform the alkali treatment, followed bywashing with pure water several times and then drying with air blowing.

Thus, a chemically strengthened glass of Example 1 was obtained.

Example 2

A chemically strengthened glass was produced in the same manner as inExample 1, except that in the polishing step, cerium oxide having anaverage particle diameter of 0.9 μm was used as the polishing agent; andthat the polishing pad was changed to a polishing pad of a suede type.

Example 3

A chemically strengthened glass was produced in the same manner as inExample 2, except that in the polishing step, the polishing pad waschanged to a polishing pad of a hard urethane type; and that in thechemical strengthening step, the sodium amount in the molten salt, thechemical strengthening treatment temperature, and the chemicalstrengthening treatment time were changed to values shown in Table 1,respectively.

Comparative Example 1

A chemically strengthened glass was produced in the same manner as inExample 1, except that in the chemical strengthening step, the sodiumamount in the molten salt was changed to a value shown in Table 1, andthe addition amount of potassium carbonate was changed to 0 g; and thatthe acid treatment step and the alkali treatment step were omitted.

Comparative Example 2

A chemically strengthened glass was produced in the same manner as inExample 2, except that in the polishing step, cerium oxide having anaverage particle diameter of 0.7 μm was used as the polishing agent;that in the chemical strengthening step, the sodium amount in the moltensalt was change to a value shown in Table 1, and the addition amount ofpotassium carbonate was changed to 0 g; and that the acid treatment stepand the alkali treatment step were omitted.

Comparative Example 3

A chemically strengthened glass was produced in the same manner as inExample 2, except that in the chemical strengthening step, the sodiumamount in the molten salt was change to a value shown in Table 1, andthe addition amount of potassium carbonate was changed to 0 g; and thatthe acid treatment step and the alkali treatment step were omitted.

Reference Example 1

A chemically strengthened glass was produced in the same manner as inExample 2, except that in the polishing step, cerium oxide having anaverage particle diameter of 0.7 μm was used as the polishing agent.

Reference Example 2

A chemically strengthened glass was produced in the same manner as inExample 2, except that in the polishing step, cerium oxide having anaverage particle diameter of 0.7 μm was used as the polishing agent; andin the chemical strengthening step, the sodium amount in the moltensalt, the chemical strengthening treatment temperature, and the chemicalstrengthening treatment time were changed to values shown in Table 1,respectively.

The thus-obtained chemically strengthened glass was evaluated forvarious properties. The results are shown in Table 1.

FIGS. 3 to 5 show graphs in which the hydrogen concentration profile inthe surface layer of the respective chemically strengthened glassesobtained in the respective Examples and Comparative Examples wasplotted.

Furthermore, FIGS. 6 to 13 show images observed by AFM with respect toglass surfaces before and after the acid treatment and the alkalitreatment of the respective chemically strengthened glasses obtained inthe respective Examples, Comparative Examples, and Reference Examples.

Further, FIG. 14 shows a Weibull plot of BOR surface strength evaluationof each of chemically strengthened glasses obtained in Example 1 andComparative Example 1. FIG. 14 shows a Weibull plot of BOR surfacestrength evaluation of an aluminosilicate glass sheet sample having athickness of 0.70 mm. The horizontal axis of the graph indicates alogarithm ln (σ) of the fracture load σ (N), and the vertical axisthereof indicates a cumulative fracture probability percentage P (%)relative to the sample in each of the two groups.

TABLE 1 Comparative Comparative Comparative Reference Reference Example1 Example 2 Example 3 Example 1 Example 2 Example 3 Example 1 Example 2Type of Glass alumino- alumino- alumino- alumino- alumino- alumino-alumino- alumino- silicate silicate silicate silicate silicate silicatesilicate silicate glass glass glass glass glass glass glass glassPolishing before chemical strengthening Performed Performed PerformedPerformed Performed Performed Performed Performed Polishing flawsPresent Present Present Present Present Present Absent Absent ChemicalK₂CO₃ mol % 6 6 6 0 0 0 6 6 strengthening Na amount wt ppm 10,000 10,0006,000 2,000 2,000 2,000 10,000 6,000 Temperature ° C. 450 450 430 450450 450 450 430 Time min 120 120 360 120 120 120 120 360 Acid treatmentChemical liquid type HNO₃ HNO₃ HNO₃ — — — HNO₃ HNO₃ Concentration wt % 66 6 — — — 6 6 Temperature ° C. 40 40 40 — — — 40 40 Time sec 120 120 120— — — 120 120 Alkali treatment Chemical liquid type NaOH NaOH NaOH — — —NaOH NaOH Concentration wt % 4 4 4 — — — 4 4 Temperature ° C. 40 40 40 —— — 40 40 Time sec 120 120 120 — — — 120 120 Sheet thickness (t) mm 0.700.70 0.51 0.70 0.70 0.70 0.70 0.51 Surface Strength (F) N 1303 1305 563564 483 485 1291 798 F/t² N/mm² 2658 2664 2196 1151 985 989 2635 3020Surface removal amount nm 189 207 212 — — — 198 210 Texture directionindex (Stdi) 0.51 0.84 0.30 0.24 0.40 0.46 0.88 0.80 Texture aspectratio (Str20) 0.37 0.66 0.04 0.05 0.50 0.33 0.50 1.00 Equation (I) a−0.136 −0.138 −0.235 −0.305 −0.379 −0.355 −0.195 −0.253 b 0.059 0.0930.125 0.148 0.184 0.187 0.093 0.131 CS MPa 877 871 922 949 939 932 870928 DOL μm 28 28 40 27 27 28 28 40

From the results in Table 1, Examples 1 to 3 each having a texturedirection index (Stdi) of 0.30 or more and satisfying the relationalequation (I) were significantly improved in the surface strength ascompared to Comparative Examples 1 to 3. In addition, Examples 1 and 2each having a texture aspect ratio (Str20) of 0.10 or more were moreimproved in the surface strength than Example 3.

In addition, from the results of FIG. 14, the average fracture load was1,303 N in Example 1, whereas it was 564 N in Comparative Example 1. The10% fracture load (B10) was 1,128 N in Example 1, whereas it was 488 Nin Comparative Example 1; and the 1% fracture load (B1) was 927 N inExample 1, whereas it was 402 N in Comparative Example 1. It can be seenfrom these results that Example 1 does not produce low-strengthproducts, and the products obtained have greatly improved reliabilityfor the surface strength.

While the invention has been described in detail and with reference tospecific embodiments thereof, it will be apparent to one skilled in theart that various changes and modifications can be made therein withoutdeparting from the spirit and scope thereof. The present application isbased on a Japanese patent application filed on Jul. 19, 2013 (JapanesePatent Application No. 2013-151116), and the content thereof is hereinincorporated by reference.

INDUSTRIAL APPLICABILITY

According to the present invention, a chemically strengthened glasswhose surface strength has been dramatically improved can be obtainedsafely and inexpensively. The chemically strengthened glass of thepresent invention is usable as a cover glass for displays such as mobilephones, digital cameras, and touch panel displays.

DESCRIPTION OF REFERENCE NUMERALS AND SIGNS

-   -   10: Low-density layer    -   20: Compressive stress layer    -   30: Interlayer

1. A chemically strengthened glass having a compressive stress layerformed in a surface layer thereof according to an ion exchange method,wherein a surface of the glass has polishing flaws, the glass has atexture direction index (Stdi) of 0.30 or more, a hydrogen concentrationY in a region to a depth X from an outermost surface of the glasssatisfies the following relational equation (I) at X=from 0.1 to 0.4(μm), and a surface strength F (N) measured by a ball-on-ring test underthe following conditions is (F≧1400×t²) relative to a sheet thickness t(mm) of the glass:Y=aX+b  (1) in which meanings of respective symbols in the equation (I)are as follows: Y: hydrogen concentration (as H₂O, mol/L); X: depth fromthe outermost surface of the glass (μm), a: −0.300 or more; and b: 0.220or less, the conditions of the ball-on-ring test: a sheet of the glasshaving the sheet thickness t (mm) is disposed on a stainless ring whosediameter is 30 mm and whose contact part has a roundness with acurvature radius of 2.5 mm; while a steel ball having a diameter of 10mm is kept in contact with the sheet of the glass, a center of the ringis subjected to a load by the ball under a static loading condition; anda fracture load (unit: N) at which the glass is fractured is taken as aBOR surface strength and an average value of twenty measured valuesthereof is taken as the surface strength F, provided that in a casewhere a fracture origin of the glass is separated from a loading pointof the ball by 2 mm or more, the obtained value is excluded from datafor calculating the average value.
 2. The chemically strengthened glassaccording to claim 1, wherein the glass is an aluminosilicate glass, analuminoborosilicate glass or a soda-lime glass.
 3. The chemicallystrengthened glass according to claim 1, having a texture aspect ratio(Str20) of 0.10 or more.
 4. The chemically strengthened glass accordingto claim 1, having a thickness of 5 mm or less.
 5. A method forproducing a chemically strengthened glass, comprising a step of bringinga glass containing sodium into contact with an inorganic salt containingpotassium nitrate, thereby performing ion exchange of a Na ion in theglass with a K ion in the inorganic salt, wherein the inorganic saltcontains at least one salt selected from the group consisting of K₂CO₃,Na₂CO₃, KHCO₃, NaHCO₃, K₃PO₄, Na₃PO₄, K₂SO₄, Na₂SO₄, KOH and NaOH, andthe method comprises: a step of polishing a surface of the glass beforethe ion exchange; a step of washing the glass after the ion exchange; astep of subjecting the glass to an acid treatment after the washing; anda step of subjecting the glass to an alkali treatment after the acidtreatment.
 6. The method for producing a chemically strengthened glassaccording to claim 5, wherein the inorganic salt has an Na concentrationof 500 ppm by weight or more.
 7. The method for producing a chemicallystrengthened glass according to claim 5, wherein the inorganic saltcontains K₂CO₃ in an amount of 0.1 mol % or more.
 8. The method forproducing a chemically strengthened glass according to claim 5,comprising a step of washing the glass after the acid treatment.
 9. Themethod for producing a chemically strengthened glass according to claim5, comprising a step of washing the glass after the alkali treatment.10. The method for producing a chemically strengthened glass accordingto claim 5, wherein a solution having a pH of less than 7 is used in theacid treatment.
 11. The method for producing a chemically strengthenedglass according to claim 10, wherein the solution having the pH of lessthan 7 is a weak acid.
 12. The method for producing a chemicallystrengthened glass according to claim 10, wherein the solution havingthe pH of less than 7 is a strong acid.
 13. The method for producing achemically strengthened glass according to claim 5, wherein the acidtreatment is performed at a temperature of 100° C. or lower.
 14. Themethod for producing a chemically strengthened glass according to claim5, wherein a time for performing the acid treatment is from 10 secondsto 5 hours.
 15. The method for producing a chemically strengthened glassaccording to claim 10, wherein the solution having the pH of less than 7has a concentration of from 0.1 wt % to 20 wt %.
 16. The method forproducing a chemically strengthened glass according to claim 5, whereina solution having a pH of more than 7 is used in the alkali treatment.17. The method for producing a chemically strengthened glass accordingto claim 16, wherein the solution having the pH of more than 7 is a weakbase.
 18. The method for producing a chemically strengthened glassaccording to claim 16, wherein the solution having the pH of more than 7is a strong base.
 19. The method for producing a chemically strengthenedglass according to claim 5, wherein the alkali treatment is performed ata temperature of from 0 to 100° C.
 20. The method for producing achemically strengthened glass according to claim 5, wherein a time forperforming the alkali treatment is from 10 seconds to 5 hours.
 21. Themethod for producing a chemically strengthened glass according to claim16, wherein the solution having the pH of more than 7 has aconcentration of from 0.1 wt % to 20 wt %.